Engine combustion of fuel may produce regulated emissions of hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx) as byproducts. NOx is a known contributor to greenhouse gases. Efforts to mitigate NOx production and/or release from internal combustion engine vehicles include exhaust gas recirculation (EGR), adjusting combustion parameters (e.g., spark timing, air/fuel ratio, and the like), and inclusion of various catalytically active aftertreatment device.
While warmed-up (e.g., lit-off) aftertreatment devices may sufficiently treat regulated emissions like NOx during a wide variety of engine operating conditions, these devices may function only above an activation temperature and/or light-off temperature, and this performance degrades from customer in-use drive cycles throughout their useful life. In one example, the useful lift of such devices may be 150,000 miles or some other the threshold number of miles. Beyond the threshold number of miles, NOx output may exceed a government standard. Specifically, palladium (Pd) may sinter under high temperatures and fuel-rich conditions, causing the vast initial coverage of Pd atoms to coalesce into large particles, thereby burying Pd atoms into the cores of these large particles where they may no longer be accessible to the gas phase. Once this occurs, aftertreatment devices demand much higher temperatures to convert engine-out emissions of NOx and other combustion byproducts, thereby lengthening the duration when regulated emissions escape out the tailpipe when first starting an engine that is at ambient temperature and undergoing an engine cold-start. Said another way, following the threshold number of miles, the devices may demand a greater duration of time to reach their activation temperature, which may prolong the engine cold-start, thereby increasing emissions. The capability of engine aftertreatment to treat emissions during the cold-start period essentially determines the government certified tailpipe emissions standard that the vehicle will qualify to be sold under. From the 2015 model year to the 2025 model year, the LEV-III regulations mandate that fleet average sum of HC and NOx tailpipe emissions must be lowered by 70% on the federal test procedure drive cycle.
Current solutions to store and convert cold-start HC emissions may include the HC trap and Passive NOx Adsorber (PNA), while current solutions to store and convert NOx emissions may include the Lean NOx Trap (LNT) and Passive NOx Adsorber (PNA). These devices each have their merits and drawbacks. Some HC trap devices may be based on a monolithic substrate coated first with an aluminosilicate zeolite that can store HC molecules and then coated second with a three-way catalyst material to convert inlet emissions like a catalytic converter, but it also treats the stored cold-start emissions in the zeolite that are later released at higher temperature. Unfortunately, conventional HC traps do not have high efficiency for NOx storage, preventing use of an HC trap as a comprehensive cold start emissions solution.
The PNA is zeolite-based similar to the HC trap, except that the TWC overcoat is optional and the zeolite contains a significant amount of ion exchanged precious metals (e.g., Pd) to replace the abundant weak physical adsorption (physisorption) sites in the zeolite for strong chemical adsorption (chemisorption) and high temperature emissions storage of HC and NOx. PNA devices are designed for lean (diesel) environments that may not be exposed to excessively hot exhaust or prolonged fuel-rich modes. Pd has been observed to go from Pd2+ (desired ionic state) to Pd0 (vulnerable metallic state) as a function of temperature and oxygen concentration. This transition can occur on bulk PdO in 21% O2 at about 800° C., in 1% O2 at about 690° C. and in 0.001% O2 at about 500° C. Thus, reduction of the Pd may occur at lower temperatures in the presence of less oxygen. When reductant (i.e., CO, HC, H2) is present and exceeds the gas-phase oxidants (O2, H2O), and PdO is warmed-up enough to be thermally active (i.e., 200° C.), then Pd metal is formed (e.g., Pd0). Continued exposure to sufficiently hot and/or fuel-rich exhaust with Pd metal in the zeolite can rapidly sinter the Pd atoms into large particles and deactivate the PNA. Hot exhaust and fuel-rich operation is typical of stoichiometric (gasoline) exhaust relative to lean (diesel) exhaust, thereby limiting the usefulness of a PNA as an effective cold start emissions solution for gasoline fuel.
The LNT may comprise platinum (Pt) for NO oxidation to NO2, barium (Ba) base metal for NO2 storage, and Rh for NO2 reduction to N2 during brief periodic fuel-rich excursions. Pt oxidizes the inlet NO to NO2 when warmed to above an activation temperature of about 150° C. In some examples, over 90% of NOx emissions on a gasoline vehicle may be in the form of NO. Ba stores inlet NO2 as Ba(NO3)2 until about 400° C. when Ba(NO3)2 starts to become thermodynamically unfavorable, causing an equilibrium shift that begins releasing stored NOx as temperature increases further. The NO oxidation step may be the critical first step of NOx emissions capture by an LNT as Ba does not store NO emissions. Therefore, the LNT is bound by kinetics on the low temperature side and thermodynamic equilibrium on the high temperature side, preventing use of an LNT as an effective cold start emissions solution.
However, the inventors have come up with a solution to at least partially address the above-described issues for stoichiometric exhaust gas NOx treatment during the cold-start period. In one example, the issues described above may be addressed by a system comprising a first catalyst arranged upstream of a second catalyst in a vehicle far-underbody and a controller having computer-readable instructions stored thereon that when executed enable the controller to adjust oxygen flow to the first and second catalysts via adjusting intake valves of one or more engine cylinders during one or more of a fuel cut and engine shut-off. In this way, preconditioning of the first and second catalysts may be optimized to promote increased emissions reduction.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.